Transistor ignition system having ballast resistor shunt to maintain constant current through the ignition transformer



Dec. 27, 1966 M. s. FlSHER ETAL 3,295,014

TRANSISTOR IGNITION SYSTEM HAVING BALLAST RESISTOR SHUNT TO MAINTAIN CONSTANT CURRENT THROUGH THE IGNITION TRANSFORMER Filed Dec. 31, 19625 6 7 3 9 A? /Z L; 1 4

mrrz/z'r mum; 64 1 0175) INVENTO Aim/44:4 5. ICISI/EA BY FKEDEK/CK .5 (AMP ilnited States atent M 3,295,014 TRANSISTOR IGNITION SYSTEM HAVING BAL- LAST RESISTOR SHUNT TO MAINTAIN CON- STANT CURRENT THROUGH THE IGNTTION TRANSFORMER Michael S. Fisher, Ringoes, and Frederick S. Kamp, Somerville, N.J., assignors to Radio Corporation of America, a corporation of Delaware Filed Dec. 31, 1963, Ser. No. 334,755 5 Claims. (Cl. 315-21S) This invention relates generally to ignition systems, and more particularly to an improved, transistorized ignition system.

It has been proposed to employ a transistor in an automobile ignition system, for example, for controlling current from a battery to the primary winding of an ignition transformer. While such an ignition system operates satisfactorily when the battery is in good condition, it leaves something to be desired when the battery is weak, discharged, and/or relatively cold. When, for example, a heavy load is placed on a 12-volt battery, as when an automobile engine is being started, the voltage of the battery may drop to 8 volts because of the voltage drop within the battery due to its internal resistance. Also, at temperatures below freezing, the voltage available from a battery is relatively lower than its normal related voltage. Under the latter conditions, the current available in the ignition system may be less than necessary or desirable for firing the spark plugs.

It is an object of the present invention to provide an improved, transistorized ignition system having means for regulating the current automatically to the ignition transformer over a relatively wide range of battery voltages.

Another object of the present invention is to provide an improved, transistorized ignition system wherein the time for current to reach a steady state in the ignition transformer is faster than in many prior art ignition systems.

Still another object of the present invention is to provide an improved, transistorized ignition system of the type described that is relatively simple in circuitry, easy to manufacture and operate, and yet highly eliicient in use.

Briefly stated, the improved ignition system of the present invention comprises means for regulating the current to the primary winding of an ignition transformer over a relatively wide range of available voltages. Current from a unidirectional source of voltage is caused to flow to the primary Winding of the ignition transformer through a ballast resistor and the emitter-collector path of a transistor. The base of the transistor is biased so as to cause current to flow to the primary winding when a pair of breaker points in the emitter-base circuit of the transistor is closed. Current regulation means are provided to shunt the ballast resistor automatically with a circuit having an impedance that varies with the voltage of the voltage source, thereby tending to maintain the current through the transistor substantially constant. Improved transistor biasing means are also employed to interrupt current through the transistor faster than in prior art ignition systems and to prevent the transistor from conducting again until the proper time in the cycle.

The novel features of the present invention, both as to its organization and method of operation, as well as additional objects and advantages thereof, will be more readily understood from the following description, when read in connection with the accompanying drawing in which:

FIG. 1 is a schematic diagram of a transistorized igni- Patented Dec. 27, 1966 tion system in accordance with the present invention; and

FIG. 2 is a graph of operating current vs. battery voltage to explain the operation of the ignition system illustrated in FIG. 1.

Referring, now, particularly to FIG. 1 of the drawing, there is shown a transistorized ignition system 10 of the type wherein the voltage from a unidirectional voltage source, such as a battery 12, supplies the energy for producing a spark across the gap 14 of a spark plug SP. The transistorized ignition system 10 may be of the type used in an automobile wherein the voltage of the battery 12 may be rated at 12 volts, for example. The negative terminal of the battery 12 may be connected to a common connection, such as ground, and the positive terminal of the battery 12 is connected to the emitter of a PNP transistor T1 through an ignition switch 16 and a ballast resistor 18. The collector of the transistor T1 is connected to the common connection through the primary winding 20 of an ignition coil or transformer 22.

One end of the secondary Winding 24 of the ignition transformer 22 is connected to the common connection, and the other end is connected to the rotor 26 of a distributor 28. A contact 30 of the distributor 28 is connected to one point 32 of the spark plug SP, another point 34 of the spark plug SP being connected to the common connection. While only one spark plug SP is shown and described herein for the sake of clarity, there is a different one for each contact of the distributor 28.

Means are provided to bias the transistor T1 into conduction when breaker points 36 and 38 are closed. To this end, the point 38 is connected to the com-mon connection, and the point 35 is connected to the base of the transistor T1 through a resistor 40 and the primary winding 42 of transformer 44. The base of the transistor T1 is connected to the emitter of the transistor T1 through the secondary winding 46 of the transformer 44 and a resistor 48. The primary winding 20 of the transformer 22 is shunted by a capacitor 49. The rotor 26 is mechanically coupled to a rotating cam 35, as indicated by the dashed line 37, to separate (open) the normally closed breaker points 36 and 38 to fire the spark plugs SP at the proper time in the cycle.

In operation, when the breaker points 36 and 38 are closed, current flows from the positive terminal of the battery 12 to the common terminal, ground, through the series circuit comprising the closed switch 16, the ballast resistor 18, the emitter-base path of the transistor T1, the primary winding 42 of the transformer 44, and the resistor 40. The voltages produced by the magnetic fluxes in the primary and secondary windings 42 and 46, respectively, of the transformer 44 are additive. Current also flows from the battery 12 to the common terminal through the closed switch 16, the ballast resistor 18, the emitter-collector path of the transistor T1, and the primary winding 20 of the ignition transformer 22.

When the cam 35 separates (opens) the breaker points 36 and 38, current ceases to flow through the emitter-base circuit of the transistor T1, and the magnetic field about the primary Winding 42 of the transformer 44 collapses. This action induces a voltage in the secondary Winding 46 of the transformer 44 in a direction such that the voltage at the base of the transistor T1 biases this transistor in a reverse direction, causing the transistor T1 to cease conducting in a faster period of time than it would without the transformer 44.

When current through the emitter-base circuit of the transistor T1 is interrupted, current ceases to flow through the emitter-collector path of the transistor T1, interrupting current through the primary winding 20 of the transformer 22 and causing the magnetic field about the primary Winding 20 to collapse. The collapsing magnetic field about the primary winding 20 causes a relatively high voltage to be induced across the secondary winding 24 of the ignition transformer 22, and current flows to the spark plug SP. In the position of the rotor 26 shown in FIG. 1, current flows from the secondary winding 24 to ground through the rotor 26, the contact 30, and the gap 14 of the spark plug SP. The back-biasing voltage produced by the secondary winding 4-6 of the transformer 44, when the breaker points 36 and 38 are separated, is of an amplitude and duration sufficient to hold off conductlon through the transistor T1 until the spark plug SP has been fired.

The ignition system It} thus far described operates satisfactorily as long as the voltage of the battery 12 is substantially constant. When the voltage of the battery 12 is lowered, as by the heavy load caused by starting an automobile engine or by excessively cold weather, sufficient current may not be supplied to the primary winding 20 to cause the spark plugs to be fired. Regulatory means, comprising the circuit included within the dashed polygon 50, are provided to maintain the current through the primary winding 20 of the ignition transformer 22 substantially constant over a relatively wide range of battery voltages. A regulatory resistor 52 is connected in shunt, that is, in parallel, with the ballast resistor 13 through the emitter-collector path of a PNP transistor T2. The base of the transistor T2 is connected to the common connection through a biasing resistor 54. The transistor T2 is biased to conduct fully, that is, to saturate, when the battery voltage is at or below a predetermined voltage, say, for example, 8 volts. Under these conditions, the resistance of the current path to that circuit including the emitter-collector path of the transistor T1 and the primary winding 20 of the ignition transformer 22 is relatively low because current can flow through the path provided by the ballast resistor 18, and the path provided by the regulatory resistor 52 and the relatively low impedance of the fully conducting transistor T1, both paths being in parallel with each other.

When the battery voltage is greater than a predetermined value, that is, greater than 8 volts, for example, means are provided to decrease, or even interrupt, current through the regulatory resistor 52 to provide a substantially constant current to the primary winding 20 over a wide range of battery voltages. To this end, a PNP transistor T3 has its emitter connected to the junction between the ballast resistor 18 and the regulatory resistor 52 and its collector connected to the base of the transistor T2. The base of the switching transistor T3 is connected to the common junction 55 between two serially connected resistors 56 and 58 through a diode 60, the anode of the diode being connected to the base of the transistor T3. The resistors 56 and 58 comprise a voltage divider that is connected in parallel with the ballast resistor 18.

When the voltage drop across the ballast resistor 18 is greater than a predetermined value, the voltage at the base of the transistor T3 is sufiicient to bias the transistor T3 into conduction. This action increases the voltage across the resistor 54 and biases the transistor T2 with a positive-going voltage that increases its impedance. Thus, current through the transistor T2 decreases when the transistor T3 is switched on, and vice versa. When the voltage of the battery 12 is greater than a predetermined voltage, say 12 volts, the transistor T3 is biased fully on, and the transistor T2 is biased fully oil. The impedance of the emitter-collector path of the transistor T2 varies from a minimum at saturation to a substantially infinite value at cut-ofi over a range of voltages across the ballast resistor 18.

The operation of the ignition system 10, including its current regulatory circuit 5-0, will now be explained. Let it be assumed, for example, that the battery 12 is used to start an automobile engine (not shown) of which the ignition system is a part. Under this heavy load condition for the battery 12, the voltage of the battery 12, due to its internal resistance, drops to, say, 8 volts. The voltage across the resistor 56 is now insufiicient to bias the transistor T3 into conduction, and the transistor T2 conducts fully. Current from the battery 12 now flows to the primary winding 20 of the ignition transformer 22 through the parallel paths provided by the ballast resistor 18 and the regulatory resistor 52, provided the breaker points 36 and 38 are closed, so that the transistor T1 is conductive. Since the current path provided by the parallel resistors 18 and 52, including the impedance of the emitter-collector path of the transistor T2, is one of decreased resistance, sufiicient current can flow to the primary winding 29 even though the voltage of the battery 12 is lowered under engine starting conditions.

Let it now be assumed that the automobile engine has started but that the battery 12 is cold so that the voltage available from it is only 10 volts. Under these conditions, the voltage across the ballast resistor 18, that is, the voltage across the voltage divider comprising the resistors 56 and 58 also, is great enough to bias the transistor T3 into conduction. Current through the emitter-collector path of the transistor T3 increases the voltage across the resistor 54 and applies an increased positive-going bias to the base of the transistor T2, increasing the impedance of the transistor T2 and decreasing the current through the regulatory resistor 52. Thus, as the voltage of the battery 12 increases from 8 volts to 10 volts, the impedance of the transistor T2 increases so that the current available to the primary winding 20 remains substantially constant.

The current that remains substantially constant is the steady-state current, and not the current during the initial rise time of the current.

Let it now be assumed that the battery 12 is providing an output of 12 volts. Under these conditions, the transistor T3 may be turned fully on, biasing the transistor T2 fully ofi. Current to the primary winding 20 of the ignition transformer 22 now flows only through the ballast resistor 18, the regulatory resistor 52 being disconnected from the circuit by the cut-off transistor T2. Under the latter conditions, the impedance of the emitter-collector path of the transistor T2 is substantially infinite.

Referring, now, to the graph of FIG. 2, the change in current through the primary winding 20, that is, through the transistor T1, for different battery voltages of the battery 12 is shown. The current through the transistor T1 increases proportionally with the battery voltage until the latter is about substantially 8.5 volts. Up to this point, current through the transistor T1 is supplied through the ballast resistor 18 and through the regulatory resistor 52, the transistor T2 being fully on, that is, saturated. When the battery output voltage is between 8.5 and 12 volts, the current through the transistor T1 is substantially constant, that is, about 8 amps. During this period, current is supplied to the transistor T1 through the ballast resistor 18 and the regulatory resistor 52, the impedance of the conducting transistor T2 varying inversely with the amplitude of the battery voltage. For voltages in excess of 12 volts, the transistor T2 is switched off, and the current through the transistor T1 is supplied through the ballast resistor 18 only. During this period, the current is substantially proportional to the battery voltage.

It was found that the time for current to reach a steady state in the primary winding 20 of the ignition transformer 22 was faster when the regulatory circuit 50 was used than when it was omitted. For an ignition system 10 of the present invention, having circuit components with the following values, the time to reach a steady state current was about one-half that for an ignition system without the regulatory circuit 50. This shortened time makes it possible for multi-cylinder engines to run at very fast speeds (r.p.m.) without output high voltage degradation because adequate current is always available in the ignition transformer to provide a good spark at the spark plugs.

An example of an ignition system in accordance with the present invention may comprise components with the following values:

Transformer 44, a bifilar wound 1:1 transformer With an inductance of about 6 mh./winding. The core is a hlgh frequency response audio material.

The above values are merely illustrative and not to be considered in a limiting sense.

From the foregoing description, it will be apparent that there has been provided an improved ignition system wherein a substantially constant current is supplied to an ignition transformer over a relatively wide range of battery voltages. While only one example of the ignition system of the present invention has been described, variations in its circuitry and applications, all coming within the spirit of this invention, will, no doubt, readily suggest themselves to those skilled in the art. Hence, it is desired that the foregoing shall be considered as illustrative and not in a limiting sense.

What is claimed is:

1. In an ignition system adapted to be energized from a voltage source which is subject to variation in output voltage, said voltage source having a pair of output terminals,

a transistor having an emitter, a collector and a base,

an ignition transformer having a primary winding and a secondary winding, a ballast resistor, means connecting said ballast resistor, the emitter to collector path of said transistor and the primary winding of said ignition transformer in series between said output terminals,

a switching means,

means for connecting the base of said transistor to a terminal of said source through said switching means, and

means for maintaining the maximum current flow through said primary winding substantially constant regardless of the voltage of said source over a predetermined range of voltage variation thereof and for applying substantially the total voltage of said source across said primary winding during the current buildup therethrough, comprising a variable conductivity shunting means connected across said ballast resistor, means to sense the voltage appearing across said ballast resistor, and means to so vary the conductivity of said shunting means in response to the voltage appearing across said ballast resistor as to keep the maximum current flow through said primary winding constant over a predetermined range of variation of said source.

2. The invention as claimed in claim 1 in which said variable shunting means comprises the emitter to collector path of a second transistor,

said voltage sensing means comprises a voltage divider having an intermediate point and being connected across said ballast resistor, and

said means to vary the conductivity of said shunting means comprises a connection from said intermediate point to the base of said second transistor.

3. An ignition system comprising, in combination,

(a) a ballast resistor,

(b) a first transistor having an emitter-collector path and a base,

(c) an ignition transformer having a primary winding and a secondary winding,

(d) two normally closed breaker points,

(e) a second transformer having a primary winding and a secondary winding,

(f) means connecting said ballast resistor, said emittercollector path of said first transistor, and said primary winding of said ignition transformer in series in a first circuit,

(g) means including a pair of terminals to apply a voltage source across said first circuit,

(h) means connecting said primary winding of said second transformer and said breaker points in series in a second circuit between said base of said first transistor and one of said terminals,

(i) means connecting said secondary winding of said second transformer between the emitter and said base of said first transistor,

(j) second and third transistors each having an emitter-collector path and a base,

(k) means connecting said emitter-collector path of said second transistor across said ballast resistor,

I (1) means connecting said emitter-collector path of said third transistor between said terminals,

(m) sampling means to sample the voltage across said ballast resistor,

(n) means connecting said base of said third transistor to said sampling means, and

(0) means connecting said base of said second transistor to said emitter-collector path of said third transistor.

4. An ignition system for causing the firing of a spark plug in an internal combustion engine, said ignition system comprising (a) a source of voltage having a pair of output terminals,

(b) means connecting one of said output terminals to a common connection,

(c) a first and a second transistor each having an emitter, a collector, and a base,

(d) a first and a second transformer each having a primary winding and a secondary winding,

(e) means connecting said secondary winding of said first transformer to said spark plug,

(f) a pair of breaker points operated in synchronism with said engine,

(g) a ballast resistor,

(h) means connecting said ballast resistor, the emittercollector path of said first transistor, and the primary winding of said first transformer in series in a circuit between said pair of output terminals,

(i) means connecting said primary winding of said second transformer and said breaker points between said base of said first transistor and said common connection,

(j) means connecting said secondary winding of said second transformer between said emitter and said base of said first transistor,

(k) a second resistor,

(1) means connecting said second resistor and the emitter-collector path of said second transistor in series in a circuit in parallel with said ballast resistor, and

(in) means to sense the voltage across said ballast resistor and to feed back signals to said base of said second transistor to vary its impedance in accordance with changes in the voltage across said ballast resistor.

5. In combination, an ignition system adapted to be energized from a battery having a pair of output terminals, one of said terminals being connected to a common connection, said ignition system comprising (a) first, second, and third transistors each having an emitter, a collector and a base,

(b) first and second transformers each having a primary winding and a secondary winding,

() means connecting one end of said primary winding of said first transformer to said common connection and to one end of said secondary winding of said first transformer,

(d) a distributor having a rotor,

(e) means connecting the other end of said secondary winding of said first transformer to said rotor,

(f) spark plugs,

(g) means connecting said rotor periodically to said spark plugs,

(h) a ballast resistor,

(i) means connecting said ballast resistor, the emittercollector path of said first transistor and said primary winding of said first transformer in series between said output terminals,

(j) a pair of breaker points,

(k) means connecting said base of said first transistor through the primary winding of said second transformer and through said breaker points in series to said common connection when said points are touch- (1) means for connecting the secondary winding of said second transformer between the base and emitter of said first transistor,

(in) means connecting the emitter-collector path of said second transistor across said ballast resistor,

(n) means connecting the emitter-collector path of said third transistor between the other of said output terminals and said base of said second transistor,

(0) means to bias said second transistor into conduction,

(p) a voltage divider connected across said ballast resistor, and

(q) means connecting said base of said third transistor to the voltage divider to bias said third transistor into conduction when the voltage across said ballast resistor is greater than a first predetermined value, said second transistor being biased to cut off when the voltage across said ballast resistor is greater than a second predetermined value.

References Cited by the Examiner UNITED STATES PATENTS 2,984,766 5/1961 Moore 315-214 3,019,782 2/1962 Kuritza 315-212 X 3,136,939 6/1964 Scott 323-22 3,150,285 9/1964 Mieras 315-209 3,152,296 10/1964 Meszaros 323-22 3,174,094 3/1965 FarnsWorth 323-22 3,219,876 11/1965 Bays et al 315-209 OTHER REFERENCES Oldham, Ford Transistor Ignition System, Electronics World, April 1963, pp. 46, 47 and 75.

JAMES W. LAWRENCE, Primary Examiner.

R. JUDD, R. SEGAL, Assistant Examiners. 

1. IN AN IGNITION SYSTEM ADAPTED TO BE ENERGIZED FROM A VOLTAGE SOURCE WHICH IS SUBJECT TO VARIATION IN OUTPUT VOLTAGE, SAID VOLTAGE SOURCE HAVING A PAIR OF OUTPUT TERMINALS, A TRANSISTOR HAVING AN EMITTER, A COLLECTOR AND A BASEAN IGNITION TRANSFORMER HAVING A PRIMARY WINDING AND A SECONDARY WINDING, A BALLAST RESISTOR, MEANS CONNECTING SAID BALLAST RESISTOR, THE EMITTER TO COLLECTOR PATH OF SAID TRANSISTOR AND THE PRIMARY WINDING OF SAID IGNITION TRANSFORMER IN SERIES BETWEEN SAID OUTPUT TERMINALS, A SWITCHING MEANS, MEANS FOR CONNECTING THE BASE OF SAID TRANSISTOR TO A TERMINAL OF SAID SOURCE THROUGH SAID SWITCHING MEANS, AND MEANS FOR MAINTAINING THE MAXIMUM CURRENT FLOW THROUGH SAID PRIMARY WINDING SUBSTANTIALLY CONSTANT REGARDLESS OF THE VOLTAGE OF SAID SOURCE OVER A PREDETERMINED RANGE OF VOLTAGE VARIATION THEREOF AND FOR APPLYING SUBSTANTIALLY THE TOTAL VOLTAGE OF SAID SOURCE ACROSS SAID PRIMARY WINDING DURING THE CURRENT BUILDUP THERETHROUGH, COMPRISING A VARIABLE CONDUCTIVITY SHUNTING MEANS CONNECTED ACROSS SAID BALLAST RESISTOR, MEANS TO SENSE THE VOLTAGE APPEARING ACROSS SAID BALLAST RESISTOR, AND MEANS TO SO VARY THE CONDUCTIVITY OF SAID SHUNTING MEANS IN RESPONSE TO THE VOLTAGE APPEARING ACROSS SAID BALLAST RESISTOR AS TO KEEP THE MAXIMUM CURRENT FLOW THROUGH SAID PRIMARY WINDING CONSTANT OVER A PREDETERMINED RANGE OF VARIATION OF SAID SOURCE. 